1. Technical Field
This invention relates to strong, low basis weight, spontaneously dispersible sheets comprising modified cellulosic fibers and to a process for producing such sheets.
2. Background Art
Numerous processes for making sheets of fibers are known in the art. The most common is, of course, the Fourdrinier process which is used extensively to make sheets of paper. Another papermaking process that has been widely recognized as being useful is that described by Sanford and Sisson in U.S. Pat. No. 3,301,746 issued Jan. 31, 1967. Sheets of paper made by any of the conventional papermaking processes or by the Sanford and Sisson process can and do exhibit wide variations in strength properties which can be closely controlled by those skilled in the art. While papers that tend to repulp on wetting can and have been made, strong sheets of fibers with truly spontaneous dispersibility in water and other liquids have been essentially unknown.
Airlaying fibers to form non-woven webs is well known in the art. These non-woven webs, which are known as airfelts and which find widespread use in a variety of products such as disposable diapers and the like, are absorbent, but are normally quite weak. These airfelts, in the absence of extraneous binders, do tend to repulp in water, but as before, lack spontaneous dispersibility.
Trokhan and Sisson in British Pat. No. 1,500,053, published Feb. 8, 1978, describe a process wherein fibers at a controlled moisture level of less than about 10% by weight of bone dry fiber are disintegrated, air laid, and compressed to form airfelts such as those used in diapers. These airfelts, as other air laid webs, lack strength and spontaneous dispersibility.
Burgeni in U.S. Pat. No. 3,017,304, issued Jan. 16, 1962, teaches a process in which an airfelt is moistened on at least one surface and then subjected to compressive forces. A paper-like skin is formed on the moistened surface. Since bonds in the skin are described as papermaking bonds, the fibers used must be those which will form bonds in the presence of moisture and pressure. From about 3% to about 13% moisture is sprayed onto the airfelt, on either one or both sides, immediately prior to subjecting the airfelt to compressive forces. Since the moisture is concentrated at the surface of the airfelt, the localized level of moisture is considerably higher than the average 3% to 13% noted. This process, then, approximates a conventional papermaking process in that fibers associated with relatively large amounts of water are brought close together by external forces and the excess water is then removed. In this particular case, the excess moisture ultimately diffuses throughout the pad thereby eliminating the necessity of deliberate drying. While this technique does increase the strength of airfelts, the strength increase is only moderate and the final products are not spontaneously dispersible in liquids. In fact, the presence of the paperlike skin on the surface of the airfelt tends to retard the normal repulping of the airfelt.
Schoggen, Holmgren and Harris in U.S. Pat. No. 3,826,711, issued July 30, 1974, describe an exemplary method of making sheets from modified cellulosic fibers. In this process, the fibers are deposited on a wire, such as a Fourdrinier wire, from a water-organic solvent slurry and then are subjected to treatment with alcohol to displace the water down to a level from about 2% to about 50% by weight prior to final drying. While the sheets of modified cellulosic fibers made by this process have a certain degree of strength, and while some of the sheets have spontaneous dispersibility if the proper modified cellulosic fibers are used, the basis weight of sheets formed by this process is of necessity relatively high. Further, the requirement of an organic solvent necessitates the use of relatively complex processing equipment which must contain solvent recovery sub-systems if it is to be economically and environmentally acceptable. Such equipment does not lend itself readily to locations other than those where the modified cellulosic are produced. That is to say, this excellent process is designed primarily for use as a step in the manufacturing process of modified cellulosic fibers.